I would recommend using a high resolution image of 300 dpi and at least 2 inches by 2 inches so that when the image is printed it is very clear.

This was from Etsy to get a necklace made.
How do I know what size my pic is and what kind of pic can I use???

i would just message them back and ask! otherwise if you go into the properties of a pic you should be able to find out but i think its a bit different on a mac than a pc so...ahem not much help!

OH is on his way home so maybe he can do it for me